Various techniques are being applied to increase the performance of semiconductor devices. One approach is to enhance the carrier mobility in current-carrying regions of a device. Carrier mobility in a silicon channel may be enhanced by altering the strain in the silicon channel. Strained silicon technology has been shown to enhance carrier mobility in both n-channel metal oxide semiconductor field effect (NMOS) transistors and p-channel metal oxide semiconductor field effect (PMOS) transistors. Enhanced carrier mobility may be used as a means to improve device speed and performance. Currently, strained silicon layers are used to increase electron mobility in n-channel complementary metal oxide semiconductor (CMOS) transistors. There has been research and development activity to increase the hole mobility of p-channel CMOS transistors using strained silicon germanium layers on silicon.
More recently, strained silicon layers have been fabricated on thicker relaxed silicon germanium (SiGe) layers to improve the mobility of electrons in NMOS transistors. Wafer bending has been used to investigate the effect of strain on mobility and distinguish between the effects of biaxial stress and uniaxial stress. Stress can also be introduced by wafer bonding. Packaging is another technique to introduce mechanical stress by bending.
Electron mobility in NMOS transistors has been improved using a tensile strained silicon layer on silicon germanium. Uniaxial compressive stress can be introduced in a channel of a PMOS transistor to improve hole mobility using silicon germanium source/drain regions in trenches adjacent to the PMOS transistor. Silicon-carbon source/drain regions in trenches adjacent to an NMOS transistor can introduce tensile stress and improve electron mobility. Silicon nitride capping layers can be formed to introduce tensile stress for NMOS transistors and can be formed to introduce compressive stress for PMOS transistors. However, there continues to be a need to provide fabrication processes and structures to enhance carrier mobility to improve performance of semiconductor devices.